Apparatus for obtaining silicon from fluosilicic acid

ABSTRACT

Apparatus for producing low cost, high purity solar grade silicon ingots in single crystal or quasi single crystal ingot form in a substantially continuous operation in a two stage reactor starting with sodium fluosilicate and a metal more electropositive than silicon (preferably sodium) in separate compartments having easy vapor transport therebetween and thermally decomposing the sodium fluosilicate to cause formation of substantially pure silicon and a metal fluoride which may be continuously separated in the melt and silicon may be directly and continuously cast from the melt.

ORIGIN OF INVENTION

The United States Government has rights in this invention pursuant toJPL/DOE Contract No. 954471-NAS 7-100 awarded by the U.S. Department ofEnergy.

BACKGROUND OF THE INVENTION

This invention together with the inventions described in the relatedapplications (cited below) evolved (in-part) from research efforts aimedat preparing low cost, high purity silicon for solar cells. The resultsof that research are contained in the following reports prepared forJPL/DOE:

Quarterly Progress Report No. 1, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur and L. Nanis, August1976;

Quarterly Progress Report No. 2 and 3, "Novel DuplexVapor-Electrochemical Method for Silicon Solar Cell", by: V. J. Kapurand L. Nanis, March 1976;

Quarterly Progress Report No. 4, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur L. Nanis, and A.Sanjurjo, January 1977;

Quarterly Progress Report No. 5, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, and A.Sanjurjo, February 1977;

Quarterly Progress Report No. 6, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, and A.Sanjurjo, March 1977;

Quarterly Progress Report No. 7, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, and A.Sanjurjo, April 1977;

Quarterly Progress Report No. 8, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, and A.Sanjurjo, February 1978;

Quarterly Progress Report No. 9, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, A. Sanjurjo,and R. Bartlett, April 1978;

Quarterly Progress Report No. 10, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. J. Kapur, L. Nanis, K. M.Sancier, and A. Sanjurjo, July 1978;

Quarterly Progress Report No. 11, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: V. Kapur, K. M. Sancier, A.Sanjurjo, S. Leach, S. Westphal, R. Bartlett, and L. Nanis, October1978;

Quarterly Progress Report No. 12, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: L. Nanis, A. Sanjurjo, and S.Westphal, January 1979;

Quarterly Progress Report No. 13, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: L. Nanis, A. Sanjurjo, K. Sancier,R. Bartlett, and S. Westphal, April 1979;

Quarterly Progress Report No. 14, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: L. Nanis, A. Sanjurjo, and K.Sancier, July 1979;

Quarterly Progress Report No. 15, "Novel Duplex Vapor-ElectrochemicalMethod for Silicon Solar Cell", by: L. Nanis, A. Sanjurjo, and K.Sancier, November 1979;

Draft Final Report, "Novel Duplex Vapor-Electrochemical Method forSilicon Solar Cell", by: L. Nanis, A. Sanjurjo, K. Sancier, and R.Bartlett, March 1980; and

Final Report, "Novel Duplex Vapor-Electrochemical Method for SiliconSolar Cell", by: L. Nanis, A. Sanjurjo, K. Sancier, and R. Bartlett,March 1980.

The subject matter of the aforementioned reports are incorporated hereinby reference.

REFERENCE TO RELATED APPLICATIONS

Other copending United States patent applications relating to thegeneral subject matter of this invention, assigned to the same assigneeand incorporated herein by reference are as follows:

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, Ser.No. 337,136 l filed Jan. 5, 1982 by Angel Sanjurjo;

Process and Apparatus for Casting Multiple Silicon Wafer Articles, (Ser.No. 453,718) filed even date herewith by Leonard Nanis;

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, (Ser.No. 453,457) filed even date herewith by Kenneth M. Sancier;

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, (Ser.No. 453,596) filed even date herewith by Kenneth M. Sancier;

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, (Ser.No. 453,337) filed even date herewith by Leonard Nanis and AngelSanjurjo; and

Process and Apparatus for Obtaining Silicon from Fluosilicic Acid, (Ser.No. 453,456) filed even date herewith by Angel Sanjurjo, now U.S. Pat.No. 4,442,082 having an issue date Apr. 10, 1984.

FIELD OF INVENTION

Silicon is, at present, the most important material in modernsemiconductor technology and is finding increased use in solar cells forthe photovoltaic generation of electricity. In view of the importance ofthe solar cell application, the stringent requirements for purity andlow cost and further in view of the orientation of the work done, theprocess and apparatus is described primarily in the context ofproduction of silicon for solar cell use. However, it is to beunderstood that both the process and apparatus used are generally usefulin the production of silicon for whatever end use, as well as othertransition metals such as Ti, Zr, Hf, V, Nb and Ta.

A major deterrent to the development of practical solar photovoltaicsystems is the cost of high purity silicon. With todays technology,approximately twenty percent of the total cost of a silicon solar cellis ascribed to the silicon material alone. That is, the cost of thesilicon material produced by the conventional hydrogen reduction ofchlorosilanes constitutes at least twenty percent of the cost ofproducing the cell. It is estimated that the cost of the silicon must bereduced by almost an order of magnitude before silicon solarphotovoltaic panels will prove to be economically feasible as a powersource. The fact that the chlorosilane processes require multipleseparations, are so energy intensive and require such large capitalinvestments indicate that cost of the silicon cannot be reducedsufficiently to make silicon solar cells economically feasible without amajor process change. As a consequence, an approach to the production ofsolar grade silicon is required which is less complex, less energyintensive and which requires less capital equipment.

TECHNICAL FIELD OF THE INVENTION

It has been found that silicon of more than sufficient purity to meetthe solar cell applications can be produced within the economicrequirements from the metallic reduction of silicon fluoride.Preferably, the silicon fluoride is generated from fluosilicic acid, alow cost waste by-product of the phosphate fertilizer industry. In thepresent invention the silicon fluoride, in the form of gaseous SiF₄, isgenerated by thermal decomposition of the fluosilicic acid and inanother system, the silicon fluoride is prepared from an aqueoussolution of fluosilicic acid by treatment with a metal fluoride whichprecipitates the corresponding fluosilicate. In the latter instance thesalt is filtered, washed, dried and thermally decomposed to produce thecorresponding silicon tetrafluoride and metal fluoride which can berecycled to the precipitation step. The silicon tetrafluoride is thenreduced by a suitable reducing metal and the products of reactions aretreated to extract the silicon. Each of the steps is described in detailusing sodium as typical reducing agent, and sodium fluoride as typicalprecipitating fluoride but the concept applies as well to other reducingmetals and metal fluorides that can reduce silicon fluoride and formfluosilicates.

The process in one form is described in detail in an article entitledSilicon by Sodium Reduction of Silicon Tetrafluoride authored by A.Sanjurjo, L. Nanis, K. Sancier, R. Bartlett and V. J. Kapur in theJournal of the Electrochemical Society Vol. 128, No. 1, January 1981. Ina form more closely allied to the present invention, a process isdescribed in an article entitled A Solar Silicon Solution? authored byScott W. Dailey in Leading Edge Summer 1979. The subject matter of botharticles is specifically incorporated herein by reference.

BACKGROUND

There are available systems for the production of silicon utilizing someof the reactions of the present system. For example, Joseph Eringer inU.S. Pat. No. 2,172,969 describes a process wherein sodiumsilico-fluoride is mixed with sodium in powder form and placed in acrucible which is heated and in the upper part of which two pieces ofcopper wire gauze are placed parallel to each other. The space betweenthe pieces of gauze, which can also be heated, is filled with copperwool. When the crucible has been filled and closed, it is heated toabout 500° C. At this temperature, reaction takes place and silicon andsodium fluoride are formed whereby the silicon which is mechanicallyexpelled by the sudden increase in pressure is collected in chambers ortowers connected to the furnace.

The equation of the reaction is as follows:

    Na.sub.2 SiF.sub.6 +4Na=Si+6NaF

or this can be expressed:

    Na.sub.2 SiF.sub.6 =SiF.sub.4 +2NaF

    SiF.sub.4 +4Na=Si+4NaF

After the reaction product has been cooled at least to 200° C. it isfinely divided and is treated with water or heat treated with dilute 1:1sulfuric acid. Hydrogen fluoride gas is liberated (which latter can thenbe made into hydrofluoric acid or a metallic fluoride) metallicsulphates are produced and the silicon separates out on the surface inamorphous form as shining metallic froth.

The reaction expressed in equation form is:

    Si+6NaF+3H.sub.2 SO.sub.4 =Si+6HF+3Na.sub.2 SO.sub.4

After the silicon has been separated from the metallic sulphatesolution, it is again washed and is dried at 80° C. The silicon obtainedin this way is in the form of an impalpable redish or grey-brown powderwhich discolors strongly and which, even if the raw products wereimpure, contains a minimum of 96-97% silicon. The yield amounts to about87% of the theoretically possible yield.

Robert Aries reports in U.S. Pat. No. 3,041,145 that attempts made toreduce silicon halides by the use of sodium vapor have not led to acommercially successful process. He gives as an example the processdiscussed in the Eringer patent, supra, and points out 96%-97% purity isentirely outside the range of purity required for silicon to be used forphotocells, semiconductor rectifiers, diodes, and various types ofelectronic equipment. As has already been discussed, the conventionalhydrogen reduction of chlorosilanes is too energy intensive to beeconomical.

Aries ascribes the purity problem to impurities in the sodium used inthe reduction reaction and teaches that further elaborate and expensivepurification of the purest available commercial grade sodium is requiredto produce silicon of solar or semiconductor grade. More recently, V. J.Kapur in U.S. Pat. No. 4,298,587 also supports the view that suchpurification is required. In fact, this patent teaches that both thesodium and the silicon tetrafluoride must be purified using a system asenergy intensive as those employed in the chlorosilane reductionprocesses.

It has been determined that silicon of the desired grade is obtainedwithout the elaborate purification of commercial grade sodium or silicontetrafluoride obtained from the fluosilicic acid (from the reactionshown above) provided the reduction reaction is carried out in such away that it goes to completion, the proper environment is maintainedduring the reduction reaction and the product is properly isolated fromcontaminating atmosphere and container walls until the reaction iscomplete and solid silicon which is below reaction temperature is formedand separated.

In copending patent applicaton entitled Process and Apparatus forObtaining Silicon from Fluosilicic Acid, Ser. No. 337,136 filed Jan. 5,1982 by Angel Sanjurjo and assigned to the present assignee, theisolation from the container is carried out using a powdered substanceso that the reaction product does not adhere and can be removed by asimple dumping process. The system is successful and can be used to aidin prevention of destruction of the silicon ingot casting cruciblecontemplated for use in the present invention. However, that arrangementgenerally is not needed in connection with the melt separation of thepresent process.

Note that Eringer mixes Na₂ SiF₆ and Na directly in performing thereaction which produces Si. It is generally thought that where bothreactants are fed together in a reaction chamber, as a result of theclose mixing, some of the impurities in the Na₂ SiF₆ are transferred tothe product silicon. This, at least in part, explains the low purity ofEringer's Si. Commercial grade Na₂ SiF₆ typically has impurities in the10 to 100 ppm wt range which make it unacceptable for production ofsolar grade silicon in most systems without further purification.Although prepurification of H₂ SiF₆ and careful precipitation of Na₂SiF₆ with pure NaF in the presence of complexing agents have yielded Na₂SiF₆ with most impurities below the 5 ppm wt level, the Al contentgenerally remains high at 20 to 30 ppm wt levels. As evidenced by bothAries and Kapur, the art would teach that not only the startingmaterials Na₂ SiF₆ and Na must be highly purified but the SiF₄ resultingfrom decomposition of the Na₂ SiF₆ must also be purified if a solargrade Si is to be provided.

The present invention is specifically concerned with performing thereaction in such a manner that the reaction can start with a relativelyimpure Na₂ SiF₆, the reaction products (Si and NaF) are formed inessentially a continuous single operation and are easily separated bymelt separation and the Si continuously cast in single crystal or quasisingle crystal ingots of solar grade.

SUMMARY AND OBJECTS OF INVENTION

In carrying out the present invention sodium fluosilicate Na₂ SiF₆ isprecipitated from fluosilicic acid followed by thermal decomposition ofthe fluosilicate to silicon tetrafluoride SiF₄. The SiF₄ is then reducedby an alkali metal, preferably Na, to obtain silicon which is separatedfrom the mix, preferably by melt separation. The reaction is carried outin a continuous procedure and in such a manner that the resultingreaction products (Si and NaF) are easily removed and separated directlyand continuously from the melt and the Si may be directly cast and grownas single crystal or quasi single crystal ingots.

The invention has for its principal object the provision of a means forobtaining silicon of sufficient purity to produce solar photovoltaiccells inexpensively enough to make their use practical.

A further object of this invention is to provide a means by whichsilicon can be obtained which is substantially free of impuritiesstarting with relatively inexpensive and impure fluosilicic acid.

A still further object of this invention is to provide a process andapparatus for producing Si wherein Na₂ SiF₆ and a reductant, preferablyNa, are introduced into compartments of a reactor which compartmentshave means for vapor transport therebetween and thermally decomposingthe Na₂ SiF₆ so that gaseous SiF₄ is transported to interact with thereductant to produce the reaction products Si and NaF.

Another object of the invention is to provide a process for producingsolar grade Si by reaction of SiF₄ and a reductant as described aboveand wherein Si is separated from the reaction products continuously anddirectly.

Still another object of the invention is to provide a process andapparatus for producing solar grade silicon as described above whereinthe silicon separated from other reaction products is cast substantiallycontinuously and directly into single crystal or quasi single crystalingots.

Still a further object of the invention is to provide process andapparatus for continuously separating Si in molten form from the moltenreaction products and casting the Si into ingots in the form of singlecrystal or quasi single crystal ingots or a continuous sheet as it isseparated.

Yet another object of the invention is to provide an inexpensive highpurity crucible for receiving Si separated from the molten reactionproducts and forming single crystal or quasi single crystal ingots.

The novel features which are believed to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof may bestbe understood by reference to the following description taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating a preferred embodiment of theprocess for producing high purity silicon by the melt process;

FIG. 2 is a graph illustrating the time, temperature and pressurecharacteristics of the silicon fluoride and sodium reaction showing timein minutes plotted along the axis of abscissae and temperature indegrees C. and pressure (torr) plotted along the axis of ordinates;

FIG. 3 is a somewhat diagrammatic central vertical section through areactor unit showing vapor transport and reaction product separationmeans and details of one embodiment of a Na₂ SiF₄ and a SiF₄ feedmechanism;

FIGS. 4 through 7, inclusive, are perspective central vertical sectionsthrough parallelepiped crucibles, according to the present invention,which may be positioned to receive and case single crystal or quasisingle crystal ingots from the Si separated by the separator of thesystem shown in FIG. 3; and

FIG. 8 is a top view looking down on the parallelepiped crucible of FIG.7 and showing the hexagonal or honeycomb structure.

DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of the process for production of pure siliconstarting with inexpensive commercial grade fluosilicic acid isillustrated in the flow diagram of FIG. 1. The overall process consistsof three major operations which encompass a series of steps. The firstmajor operation (shown in brackets 10 in the drawing) includes the stepof precipitation of sodium fluosilicate from fluosilicic acid followedby generation of silicon tetrafluoride gas. The second major operation(brackets 12 on the drawing) comprises the reduction of silicontetrafluoride to silicon, preferably by sodium, and the third operation(brackets 14) involves the separation of silicon from the mixture ofsilicon and sodium fluoride.

Consider first the steps for generation of silicon tetrafluoride(operation 10). The preferred starting source of silicon is an aqueoussolution of fluosilicic acid (H₂ SiF₆), a waste product of the phosphatefertilizer industry, that is inexpensive and available in largequantities. Fluosilicic acid of commercial grade [23 weight percent(w%)] has also been used directly as received without purification orspecial treatment and is shown as the silicon source 16 in FIG. 1. Asanother alternative, fluosilicic acid is obtained by treating silica, orsilicates (natural or artificially made) with hydrogen fluoride. TheSiF₆ ⁻² is then precipitated in sodium fluosilicate Na₂ SiF₆, by addinga sodium salt to the solution (step 18). Other salts such as NaF, NaOH,NaCl, or similar salts of the elements in groups IA and IIA of theperiodic table are all candidates. The major selection criteria are, lowsolubility of the corresponding fluosilicate, high solubility ofimpurities in the supernatant solution, high solubility of theprecipitating fluoride salt, and non-hygroscopic character of thefluosilicate.

Based on these criteria, the preferred fluosilicates in order ofpreference are Na₂ SiF₆, K₂ SiF₆ and BaSiF₆. Using the preferred NaF asthe precipitating salt, the hydrogen of the fluosilicic acid isdisplaced by the sodium to form sodium fluosilicate, a highly stable,nonhygroscopic, white powder, and sodium fluoride which is recycled. Inequation form the reaction is

    H.sub.2 SiF.sub.6 +2NaF=Na.sub.2 SiF.sub.6 +2HF

As an example, Sodium fluosilicate was precipitated by adding solidsodium fluoride directly to the as received commercial grade fluosilicicacid 18. The yield was a supernatant liquid containing mostly HF andsome NaF and H₂ SiF₆ along with the sodium fluosilicate. HF is alsogiven off (20). The supernatant fluid was removed and the sodiumfluosilicate washed with cold distilled water to remove any remaining HFand H₂ SiF₆. After filtering and drying in an oven at 200 degrees C., aminimum yield of 92% of pure sodium fluosilicate 22 (determined by x-raydiffraction) was obtained. The product sodium fluosilicate is anonhygroscopic white powder that is very stable at room temperature andthus provides an excellent means for storing the silicon source beforeit is decomposed to silicon tetrafluoride.

Precipitation under the just described conditions acts as a purificationstep, with most impurities in the original fluosilicic acid staying insolution. This effect is increased by adding suitable complexing agentsto the fluosilicic acid solution previous to the precipitation. Bothinorganic complexing agents such as ammonia and organic agents such asEDTA (ethylenediaminetetraacetic acid) help to keep transition metalions in solution during precipitation of the fluosilicate.

The fluosilicate is thermally decomposed 24, thus,

    Na.sub.2 SiF.sub.6 =SiF.sub.4 +2NaF

to give the solid sodium fluoride, which is recycled 26, and to generatethe SiF₄ gas 28. The decomposition does not take place appreciably attemperatures below 400° C. Therefore, impurities which are volatile atthis temperature can easily be removed by a vacuum treatment below thistemperature. The decomposition of Na₂ SiF₆ takes place at temperaturesbetween 500° and 700° C. Impurities left in the solid phase aretypically transition metal fluorides such as Fe, Ni, Cu, etc., whosevolatility at temperatures below 700° C. is very low and therefore donot contaminate the SiF₄ gas. The gas thus produced can be fed directlyto the reduction reactor or it can be stored for future use.

In separate experiments, it was determined that SiF₄ gas at a pressureof 0.4 atm is in equilibrium at 650° C. with solid Na₂ SiF₆ and NaF.Therefore, as SiF₄ is needed, the Na₂ SiF₆ is thermally decomposed(FIG. 1) at 65° C. in a graphite-lined, gas-tight stainless steelretort. Gaseous SiF₄ evolved at 650° C. was condensed as a white solidin a storage cylinder (cooled by liquid nitrogen) attached to theretort. The SiF₄ gas was allowed to expand by warming of the storagecylinder to room temperature and was fed into the reactor as needed.SiF₄ gas prepared in this manner was determined by mass spectrometricanalysis to be more pure than commercial grade SiF₄, as shown in TableI. Ions formed from the sample gas were identified from the observedmass numbers, isotopic distribution and threshold appearance potentials.The detection limit was better than 0.005%. Positively identifiedgaseous impurities are listed in Table I; no metallic impurities weredetected. Peaks corresponding to B compounds, such as BF₃, werespecially checked, but none were found.

                  TABLE I                                                         ______________________________________                                        Mass spectrometric analysis of SiF.sub.4                                                                (%) SiF.sub.4 commercial                            Ion     SiF.sub.4 prepared from H.sub.2 SiF.sub.6                                                       (%)                                                 ______________________________________                                        SiF.sub.3.sup.+                                                                       96.9              93.6                                                Si.sub.2 OF.sub.6.sup.+                                                               3.04              4.24                                                SiOF.sub.2.sup.+                                                                      (--)              1.79                                                CCl.sub.3.sup.+                                                                       (--)              0.159                                               SiO.sub.2 F.sub.2.sup.+                                                                0.076            0.098                                               Si.sub.2 O.sub.2 F.sub.4.sup.+                                                        (--)              0.081                                               SiO.sub.2.sup.+                                                                       (--)              0.035                                               ______________________________________                                    

Although the SiF₄ produced from H₂ SiF₆ has less impurity, thecommercial grade SiF₄ was also used for experimental convenience. Thepossible presence of metallic impurities in commercial SiF₄ wasdetermined by bubbling the gas through high purity water and treatingthe resulting slurry with an excess of HF to drive off Si as SiF₄. Thefinal clear solution was then analyzed by plasma emission spectroscopy(PES). The results are listed in Table II, together with PES analysis ofthe waste by product H₂ SiF₆ and the NaF used to precipitate Na₂ SiF₆(18 and 22 FIG. 1). Comparison of the first two columns of Table II withcolumn three shows that the concentration of some elements, e.g., Li, B.V. Mn, Co, K, and Cu, were unchanged by precipitation of Na₂ SiF₆whereas the elements Mg, Ca, Al, P, As, and Mo were diminished by afactor of 5-10. Some elements were concentrated into the Na₂ SiF₆,namely Cr, Fe, and Ni. The fourth column in Table II is representativeof the impurity content to be found in SiF₄ gas prepared on a commercialscale. The low content of P is of special significance for bothsemiconductor and solar cell applications. Elements known to reducesolar cell efficiency (V, Cr, Fe, Mo) are uniformly low in commercialgrade SiF₄. Only Mn, As, and Al are of comparable concentration in bothNa₂ SiF₆ and SiF₄ at the 1 parts per million (ppm) by weight or lesslevel.

                  TABLE II                                                        ______________________________________                                        Plasma emission spectroscopy analysis, ppm (wt)                               Element   H.sub.2 SiF.sub.6                                                                     NaF        Na.sub.2 SiF.sub.6                                                                  SiF.sub.4                                  ______________________________________                                        Li        0.1     (--)       0.2   0.01                                       Na        460     (--)       (--)  1.8                                        K         9.0     (--)       8.0   0.3                                        Mg        55      (--)       6.4   2.3                                        Ca        110     10         18    1.6                                        B         1.0     (--)       0.8   <0.01                                      Al        8.0     <2.5       1.3   1.2                                        P         33      (--)       5     0.08                                       As        8.8     (--)       0.2   0.28                                       V         0.3     <5         0.3   <0.01                                      Cr        0.8     <3.5       8.8   <0.01                                      Mn        0.2     <4         0.4   0.16                                       Fe        13      <7         38    0.04                                       Co        0.54    (--)       0.7   <0.01                                      Ni        1.17    <8         4.2   <0.01                                      Cu        0.12    <4         0.6   <0.01                                      Zn        1.4     (--)       1     <0.01                                      Pb        14.5    (--)       5     0.03                                       Mo        11      (--)       1.0   <0.01                                      ______________________________________                                         SiF.sub.4 /Na reaction, the central operation of the pure Si process,         (FIG. 1) is the reduction of SiF.sub.4 by Na according to the reaction        SiF.sub.4 (g) + 4Na(1) = Si(s) + 4NaF(s)                                 

This reaction is thermodynamically favored at room temperature, however,it has been found experimentally that Na has to be heated to about 150°C. before any appreciable reaction can be observed. Once the reactionhas been initiated the released heat raises the temperature of thereactant (Na) which in turn increases the reaction rate. Under adiabaticconditions, a temperature of 2200 K. is predicted for the reaction withthe stoichiometric quantities of SiF₄ and Na. In practical reactors,rapid consumption of gaseous SiF₄ produces a pressure decrease. Thekinetic behavior of the Na-SiF₄ reaction is complex because of theinterplay of several factors, e.g., pressure of SiF₄, vaporization ofNa, local temperature, porosity of two solid products, and transport ofSiF₄ and Na vapor through the product crust that forms on the liquid Na.

Although only preliminary studies have been made of the kinetics, thegeneral features of this reaction have been surveyed. In a series ofexperiments to estimate reaction temperature 5 grams of Na were loadedin a Ni crucible (3 cm ID, 4 ccm high) and heated in SiF₄ initially at 1atm pressure. The Na surface tarnished at around 130° C., with theformation of a thin brown film. As the temperature increased, the colorof the surface film gradually changed from light brown to brown andfinally to almost black. The SiF₄ /Na reaction became rapid at160°+/-10° C. and liberated a large amount of heat, as indicated by asudden rise in reaction temperature. See FIG. 2 for reaction time,temperature, pressure characteristics. The pressure in the reactortypically decreased slightly until the temperature increased sharply,with an associated rapid decrease in SiF₄ pressure. The reaction lastsfor several seconds only (until the Na is consumed). For SiF₄ pressuresbelow 0.3 atm the reaction mass was observed to glow at a dull red heat.For higher pressure, a characteristic flame was observed. The shortestreaction time (20 sec) and the highest temperatures (about 1400° C.)were obtained when the initial pressure of SiF₄ was around 1 atm. Inaddition, complete consumption of Na was obtained for 1 atm SiF₄. Whenscale-up of this reaction was attempted by loading larger amounts of Na,it was found that as the depth of the Na pool increased, the amount ofNa remaining unreacted also increased. The product formed a crust on topof the Na surface, building a diffusion barrier for the reactants. Asthe barrier thickness increased, the reaction slowed and eventuallystopped.

For separation (operation 14 FIG. 1) of the silicon from the products ofreduction, in the preferred melt separation process embodiment of thisinvention, the products are heated until a melt is formed and the NaF isdrained off (36) leaving the Si (34) which can if necessary be furtherpurified. The melting and separation process is described in detailbelow in connection with the scaled up system. Leach separation isdescribed in the copending Sanjurjo application previously referenced.In the leach process, the silicon and sodium are removed and combinedwith water and a selected acid. The resultant silicon and water solublesodium fluoride are then separated.

On the bases of studies of the parameters that affect the reaction andare effected by the reaction as described in connection with the workreported above, the present system was conceived. It was consideredpossible to simplify the process and apparatus by reducing the steps toa minimum and at the same time produce a high purity product (Si). Withthis in mind and to obtain Si as pure as possible an experiment wascarried out with a reactor design in which Na was loaded in a coppercylindrical cup 1.5 inches diameter by 4 inches high (not shown). Thecup was then placed in an alumina (Al₂ O₃) tube 2.5 in in diameter (notshown). Sodium fluosilicate was added in the annular space between thecopper cup and the alumina tube. Thus, the reactants were separatelyloaded in two concentric compartments, divided laterally by a copperwall. This geometry allowed for an efficient heat transfer from theinner (cup of Na) compartment to the outer (tube of Na₂ SiF₆)compartment and allowed for easy vapor transport to the sodium. Thissystem was placed in a stainless steel reactor (not shown), which wasthen evacuated and backfilled with argon. The reactor was heated to 500°C. and reaction took place in the inside compartment, as indicated by anincrease in temperature.

X-ray analysis indicated that after the reaction, the inner compartment(copper cup with Na) contained Si and NaF and that the outer compartment(alumina tube) contained NaF and Na₂ SiF₆. From this information, it wasconcluded that during the initial heating, SiF₄ (g) is generated by Na₂SiF₆ decomposition. The SiF₄ (g) reaches the liquid Na and reacts withit to produce silicon and to liberate heat. As heat is released, Na₂SiF₆ is decomposed further, generating more SiF₄ (g). The synergisticprocess continues until one of the reactants has been depleted.

The overall reaction is very fast (order of seconds) for 10 grams of Na.The pressure build-up is minimal (less than 3 atm) and is due partiallyto the thermal expansion of argon initially present at 1 atm. Inaddition, some insight into the mechanism by which the reaction takesplace has been gained by experimentally demonstrating the presence ofSiF₄ (g) as an intermediate.

It has been observed (as previously pointed out) that at SiF₄ pressuresgreater than 0.5 atm, the SiF₄ -Na reaction products obtained have thebest yield, morphology, and composition. To obtain this pressure of SiF₄from the decomposition of Na₂ SiF₆ it is necessary to heat the salt totemperatures around 700° C. At this temperature the rate ofdecomposition is very fast and thus a small residence time is requiredfor total decomposition. Finally, the formation of NaF-Na₂ SiF₆ moltenmixtures should be avoided, since it decreases the activity of thefluosilicate and thus decreases its pressure of decomposition. Thus,temperatures of decomposition should be kept near, but not above about,700° C.

This information has been used to design reactors for a continuousproduction of silicon such as the one shown in FIG. 3. The system isessentially a single continuous process for producing low cost, highpurity solar grade silicon ingots in single crystal or quasi singlecrystal ingot form in a substantially continuous operation in a twostage reactor starting with sodium fluosilicate and a reductant inseparate compartments. The separate compartments have provision for easyvapor transport therebetween. The sodium fluosilicate is decomposedthermally to cause vapor transport of SiF₄ between the compartments andthus, the formation of substantially pure silicon which may becontinuously separated from the reaction products and directly andcontinuously cast from the melt.

In addition to the advantages of the continuous process, such a systemuses the heat generated by the reaction itself to decompose the feedingsalt and to heat up the reaction products. As already pointed out thereaction is highly exothermic and the heat generated is considerable.Therefore, use of the heat generated in this manner takes advantage ofthe energy which would otherwise require external sources. A furtheradvantage is that the in situ thermal decomposition step provides apurification step for the SiF₄ because other metal fluorides are notvolatile.

A central vertical section through a two stage reactor and meltseparator according to the present invention is illustrated in FIG. 3.The upper section 40 of the reactor system, shown somewhatschematically, constitutes a reactant (Na and Na₂ SiF₆) dispenser ordelivery system and the lower section 42 is the reactor and meltseparator section where the reaction and melt separation takes place.The reactant Na can be fed into the reactor section 42 alone as pelletsor as a liquid or it can be premixed with the reactant Na₂ SiF₆ and thetwo reactants fed in together. Premixing the reactants eliminates someot the advantages described above and, therefore, is not a preferredembodiment. Consequently, in this (preferred) illustrated embodiment,the reactant dispenser section 40 is designed to feed the two reactantsinto the reaction section 42 separately. In the illustrated embodiment,provision is made for the Na to be delivered separately as relativelysmall pellets.

In order to accomplish the separate Na delivery, reactant deliverysection 40 includes a sealed stainless steel sodium pellet (43) holdingand delivery hopper 44 vertically and centrally located on the topflange 46 of the reactor. The Na holding and delivery hopper 44 isprovided with a conventional stainless steel feeding propeller 48 forcontrolling in or near a delivery throat section 49 for controlling Naflow into the reactor section 42. The inner diameter of the Na injectionthroat section 49 and injection propeller size and speed are selected toprovide the desired Na delivery. A valve (not shown) is provided thatallows the hopper 44 to be recharged while the reactor is in operation(kept under a SiF₄ atmosphere) is provided. In order to prevent hotparticles or gas from reaching the unreacted Na in the feed hopper 44, athrottle 47 is provided in the hopper feed throat section 49.

Delivery of the reactant Na₂ SiF₆ into the reactor section 42, iscarried out by a separate Na₂ SiF₆ dispenser (delivery system) 50positioned and sealed to the top flange 46 (right side in the figure).The delivery system for the Na₂ SiF₆ (51) includes a Na₂ SiF₆ holdingand dispensing feed hopper 52 positioned (upper right in drawing) on andsealed to a feed tube 54 which is provided with an internal motor (56)driven flight or screw drive 58. The opposite end (left in drawing) ofthe feed tube 54 is sealed to the top flange 46 of the reactor section42 and further is provided with an open feed throat 60 so that as themotor 56 drives the flight 58, Na₂ SiF₆ fed from the hopper 52 is drivenin the feed tube 54 toward the reactor 42 (left in the drawing) and intothe reactor section 42 through the feed throat 60. The rate at which theNa₂ SiF₆ is fed into the reaction section 42 is determined by the speedof rotation of the screw drive 58.

Next consider the structure of the reactor and melt separator section 42of the system. In keeping with the experiment described above, thereactor section 42 illustrated is designed so that the reactant Na 43delivered from the delivery throat 49 of the Na feed hopper 44 enters acompartment (first compartment) or chamber 62 in the top center of thereaction section 42. The Na receiving chamber 62 has its outer dimensiondefined by a cylindrical reactor main wall 64 which may be of highpurity graphite and which extends essentially the full height of thereactor section 42. The part of the main wall 64 surrounding the Nareceiving compartment or chamber 62 is formed with apertures or passages66 therethrough. The passages 66 are of a size to allow vapor transportbut prevent the passage of Na₂ SiF₆ or NaF either in solid or liquidform. This may be accomplished either by making this part of the reactorsection main wall 64 porous or by producing apertures of a size betweenabout 0.001 mm and about 0.1 mm.

Also in keeping with the experiment described above, the reactor section42 illustrated is designed so that the reactant Na₂ SiF₆ 51 deliveredfrom the delivery throat 60 of the Na₂ SiF₆ feed hopper 52 enters acompartment (second compartment) or chamber 68 which surrounds the topcenter of the reaction section 42. Thus, the Na_(s) SiF₆ receivingchamber (second chamber) 68 has an inner diameter defined by the upperporous portion of the main reactor wall 64 which also defines the outerdiameter of the Na receiving chamber 62. In other words, the Na and Na₂SiF₆ receiving chambers, 62 and 68 respectively, have a common wall 64which is formed with apertures or passages 66 therethrough that are of asize to allow vapor transport. The important point here is that vaportransport take place freely between the two (second and first)compartments 68 and 62.

The inner wall 64 of the Na₂ SiF₆ receiving chamber 68 (second chamber)is circumscribed by another high purity graphite cylinder 70 which isspaced therefrom and concentric therewith. The spaced and concentriccylinder 70 forms the outer wall of this (second) chamber 68. Na₂ SiF₆receiving chamber 68 is closed at its top by the top 46 of the reactorsection 42 except for the open Na₂ SiF₆ delivery throat 60 and is closedaround its bottom with an annular ring-like bottom 71 of the samematerial as the cylindrical sides 64 and 70. Drainage for liquid NaF andimpurities from the bottom of the Na₂ SiF₆ receiving chamber 68 isprovided at opposite sides of the bottom by drainage ports 72. Each ofthe drainage ports 72 is provided with pressure gages 74 and pressureregulating valves 76 (one of each shown-on drain at right side ofdrawing). If required, separate heating means such as electrical heatingcoils (not shown) are provided around the outer wall 70 of the Na₂ SiF₆receiving chamber 68 to start the thermal decomposition and produce theSiF₄ reactant.

Consider now the part of the reactor section 42 described to this point.The Na₂ SiF₆ 51 is fed in the outer (second) chamber of the reactorsection 42 where it is thermally decomposed to SiF gas and NaF liquidjust as in the experiment described above. The vapor (SiF₄) istransported through the porous inner wall 64 of the outer chamber 68into the Na receiving chamber 62 where it reacts with the Na to produceSi according to:

    SiF.sub.4 +4Na=Si+4NaF

and in the process releases 164 kcal/mol of SiF₄ which are used todecompose more Na₂ SiF₆ and heat up the products. The adiabiatictemperature for the total reaction (Na₂ SiF₆ →Si) being near 1300° K.The high temperatures produced are expected to keep the pressure of theouter chamber well above 1 atm and the reaction (equation above) willkeep the pressure of SiF₄ in the inner reacting chamber 62 below 1 atm.A SiF₄ flow from the outer to the inner chamber should protect thegraphite walls by preventing Na from reaching them.

The reduction reaction (FIG. 1 operation 12) takes place in the innerchamber 62 (first compartment) and may go to completion in the lowerpart, i.e., in the upper part of the reaction product separating chamberor compartment 78 (third compartment) which is in the lower part ofreactor section 42. The two inner compartments 62 and 78 do not have asharp defining line between them, however, it simplifies the descriptionand understanding of the process to consider the two chambers 62 and 78as separate. In any case, the reaction product separation definitely andcompletely takes place in the lower inner compartment 78 as describedbelow.

It is contemplated that the reaction products (NaF and Si) will beseparated by a melt process at temperatures above the melting point ofSi (1412° C.) and preferably about 1420° C. or at least in the rangebetween about 1415° and about 1500° C. It is also contemplated that theseparation and removal of the reaction products will take place on acontinuous basis. The physical structure and configuration of thereactor portion 42 of the system is designed to produce such results.Note that the melt separation can be accomplished as described in thecopending patent application (Ser. No. 453,456, now U.S. Pat No.4,442,082 having an issue date Apr. 10, 1984), supra., however, apreferred embodiment is illustrated here.

As illustrated, the lower part of the reactor section 42 includes adouble container or compartment arrangement with an outer generallycylindrical container 80 (fourth compartment or chamber), designed tocapture and dispense the liquid NaF reaction product, surrounding theinner cylindrical reaction product receiving and separating container78. In order to withstand the high temperatures involved and to avoidcontaminating the reaction products, the outer wall 81 of the innercontainer 78 is composed of high purity graphite and in order to performthe separation of reaction products, the container wall 81 is made withsmall continuous perforations 79. The bottom 82 of the container 78 isgenerally conical in shape with a solid nonporous cylindrical molten Siremoving drain pipe 84 in the center and thus, has the appearance of acommon funnel. The drain pipe 84 is shown closed by a movable drain plug86. The condition illustrated is for a normal run in process withreaction products built up and some melt separation products in place.

The Na and SiF₄ mix and react in the upper inner container orcompartment 62 and continue to drop into the lower inner reaction andmelt separation compartment or chamber 78. As the reaction proceeds, apool 88 of reacted and partially reacted Na and SiF₄ form where thereaction goes to completion. Immediately below the pool of reactionproducts 88, a hotter melt separation zone 90 is formed. The meltseparation zone 90 is maintained at a much higher temperature (means ofheating explained below) than the reaction products zone 88 above it andthe reaction products effectively melt out. At these temperatures, i.e.,temperatures above 1412° C. the reaction products (Si and NaF) areliquids which are separable because the NaF will normally float on topof the Si. That is, the liquid Si, which is more dense than NaF,agglomerates and settles to the bottom of the reaction productseparating chamber 78. Liquid NaF, which melts at 993° C., is immisciblewith Si and usually wets graphite in the presence of liquid Si. The moreor less spherical globules 92 of Si dispersed in NaF 88 and the large Siingot or pool 94 at the bottom, as illustrated in FIG. 3, look much likethose actually found in a sectioned graphite container after thereaction products, heated to the temperatures contemplated here, havebeen allowed to cool (solidify). It is apparent that, while molten, theNaF both coats the Si and wets the graphite, thus, providing a barrierwhich prevents the Si from reacting with the graphite and avoids anyimpurity transfer or migration through and from reactor walls.

In view of the heat requirements for melt separation of the reactionproducts, the inner melt separation chamber 78 alone can be heated. Inthe embodiment illustrated, the outer perforated graphite wall 81 iscoated with SiC and a Grafoil strip 110 is wrapped around the wall andused as an electrical heater to elevate the temperature of the reactionproducts (temperatures discussed above). Electrical connections(labeled + and - in the drawing) are provided by thick graphite rodsbrought out through the insulation 104 at the bottom of the reactorsection 42.

Due to its relatively high surface tension (relative to NaF), Si remainsin a porous or perforated container 78 while the low surface tension NaFflows out the pores or perforations 79 provided the pores are of theproper size for the temperatures of the reaction products. It has beendetermined experimentally that for the melt zone temperaturescontemplated, perforations in the wall 81 of the inner reaction productreceiving and separating container 78 of between 2 and 3.5 millimeters(mm), the NaF flows through the perforations (now shown) while themolten Si remains in the container 78. The average dimension of theperforations 79 may be from less than 0.5 mm to about 3 mm or greater,preferably between about 0.2 mm to about 3.5 mm, more preferably betweenabout 1 mm to about 3.5 mm, and most preferably between about 2 mm toabout 3.5 mm. If the perforations are appreciably smaller than 2 mm, theNaF does not discharge well unless pressure is applied and for aperturesappreciably greater than 3.5 mm Si has a tendency to enter and interferewith NaF discharge.

The Si is removed by extracting the movable closure plug 86 to allow theSi to flow out of the reaction container drain pipe 84. The flow ispreferably adjusted so that the process is continuous. That is, the flowof Si out the pipe 84 is adjusted so that the reduction reaction iscontinuously taking place in the reactor and reaction productscontinuously settle through the reaction product zone 88 and into themelt separation zone 90 with NaF continuously flowing out the perforatedinner reaction compartment 78 and Si agglomerating at the bottom andbeing continuously withdrawn from the drain pipe 84.

The generally cylindrical outer container 80 of the reactor section 42performs the functions of collecting and dispensing the NaF (separatedreaction product) and its outer wall (main wall 64) physically supportsthe inner graphite reaction product receiving and separating container78 and the insulation 104 which minimizes radiation heat loss over theentire reactor and separator section 42. The functions performed by theouter container 80 in large measure prescribe the characteristics of thematerial used and its structure. For example, the fact that thecontainer 80 collects and dispenses the NaF reaction product which seepsthrough the perforations 79 in the inner product collecting andseparating chamber 78 makes it desirable to make the the container of amaterial which will not slough off, react with the hot NaF, or in anyway introduce contaminates which would prevent the NaF from beingrecycled without being purified. It is also desirable that the outerwall (main wall 64) of outer container 80 be spaced far enough from theouter wall 81 of the inner container 78 to provide free flow for theNaF. The walls 64 and 81 of the two containers 80 and 78 are held intheir spaced relationship at the top by means of an annular graphitering 96 which snuggly surrounds the inner container 78 near its top andfits tightly inside the outer main wall 64 of container 80 and at thebottom by means of the reaction container drain pipe 84 which is sealedin the exit aperture 98 in the bottom 100 of the outer container 80. TheNaF is discharged through a drain pipe 102 at the bottom (right side indrawing) of the outer container 80. For control purposes, the NaFdischarge pipe 102 is provided with a pressure gage 106 and aconventional valve 108.

As indicated above, high purity graphite meets the criteria for outermain wall 64 of the reactor section 42. Silicon carbide (SiC) is amaterial which also meets the criteria for the container 80. If SiC isused, however, it should be lined with graphite, Si or anothernoncontaminating powder as taught in the copending Sanjurjo patentapplication (Ser. No. 337,136), supra., to assure that no contaminantsare added to the reaction products from the container walls. Beryllia(BeO) is also a high strength ceramic which meets all of the abovecriteria for the outer container 80.

The Si discharged via the exit pipe 84 may be treated in any number ofways. For example, it may be cast into wafers (see copending patentapplication (Ser. No. 453,718), or continuously cast into sheet (seecopending patent application (Ser. No. 453,456, now U.S. Pat. No.4,442,082 having an issue date Apr. 10, 1984) but, as illustrated, it iscast into single or quasi single crystal ingots by a preferredtechnique. In order to form the Si ingots, the Si exit pipe 84 (belowthe blocking plug 86) discharges the molten Si into graphite cruciblesas described below. Since the Si in the inner reaction product receivingand separating container 78 is in the presence of NaF salt and the saltwets the Si but not the graphite (as explained above) formation of SiCis prevented. As the Si is discharged through the discharge pipe 84, thesalt (NaF) coats and isolates the Si from the pipe wall throughout itslength.

The apparatus of the instant invention for casting one or more singlesilicon crystals or "quasi" single silicon crystal (ingots) comprises atleast one graphite crucible with a molten salt coating. Such a castingsystem is characterized by reduced cost, reusability, and manufacturingversatilely. The casting crucible may be of any desired shape, such as aparallelepiped (right rectangular prism, cube, hexagonal and the like).

The molten silicon 94 is directed into a high purity graphite castingcrucible 200 having a configuration of a parallelepiped (a rightrectangular prism as illustrated in FIGS. 4 and 5) and continuously castinto a single silicon crystal in accordance with one embodiment of thepresent invention.

Other suitable parallelepiped crucible configurations useful in thepresent invention may be a square doughnut shape crucible 220 (see FIG.6). The crucible 220 is constructed so that a cooling tower 211 ispositioned centrally to facilitate directional cooling of the moltensilicon. Another suitable crucible configuration useful in the instantinvention may be in the form of a multi-compartment crucible 230 withsix adjointing hexagonal parallelepipeds (see FIG. 7). Themulti-compartment crucible 230 may be constructed so that each of twoadjacent hexagonal parallelepiped share a common side wall 232 and acooling parallelepiped 236 extends up centrally within the sixparallelepipeds crucibles so as to form an assembly of hexagonal prismstructures capable of casting six single crystal or quasi single crystalingots each in the shape of a hexagonal prism. The coolingparallelepiped 236 walls are parallel to the corresponding lateral facesof adjacent crucibles and forms a hollow cavity 235 so as to providecooling from the center to the surrounding crucibles facilitatingsolidification of the molten silicon.

High purity Grafoil and/or Grafelt (not shown) can be utilized as linersproviding a flexible wall against which the solidifying silicon 206(FIGS. 6 and 7) may expand without fracturing the crucible walls.Normally silicon 94 is introduced into the crucible together with asmall amount of NaF (typically about 10% by weight). The NaF 205 and 204(FIGS. 5, 6, and 7) also serves to absorb stress between solidifyingsilicon (206) and graphite walls so as to prevent fracture of thecrucible walls during solidification of the crystal or quasi-crystal.Low melting salts (e.g., KNO₃, NaNO₃ and the like) may be mixed with theNaF to decrease the solidification temperature of the salt mixture so asto allow the crystaline ingot to be dumped at a lower temperature andfurther reduce any residual stress.

The crucible after filing is next covered by a lid and heated in afurnace by resistance or induction heating (not shown). The heatedcrucible containing silicon 206 (FIGS. 6 and 7) is kept at a temperatureof about 1420° C. for about 15 minutes and slowly cooled in stages toabout 1412° C. or below. A silicon seed 208 is placed in contact throughan orifice 202 at the bottom of the crucible (by removing a graphiteplug centrally located at the bottom of the crucible not shown).Alternatively, a silicon seed 208 embedded on a sliding graphite member(not shown) may be utilized to contact the molten silicon 202 (FIGS. 6and 7). It is contemplated that the NaF layer 204 surrounding the moltensilicon 206 may be of great advantage in the seeding process byimproving thermal contact of the seed 208 material with the moltensilicon 206 (FIGS. 6 and 7).

In accordance with the practice of the instant invention, cooling of themolten silicon 206 (FIGS. 6 and 7) will be provided by He gas flow(similar to conventional HEM methods). Casting of silicon by one workingof the (HEM) technique are described in "Heat Exchanger Method-IngotCasting/Fixed Abrasive Method-Multi-wire Slicing Phase II" Final Reportby F. Schmid, et al., Crystal Systems Inc., June 1979; and "SiliconIngot Casting-Heat Exchanger Method (HEM)/Multi-Wire Slicing-FixedAbrasive Slicing Technique (FAST) Phase IV" Quarterly Progress ReportNo. 3, by F. Schmid, et al., Crystal Systems, October 1980. The subjectmatter of the above cited publications are incorporated herein byreference.

As the solidification proceeds, fracture of the crucible is prevented bythe combination of spongy Grafoil lining the crucible wall and moltenNaF. Alternatively, a slight taper of about 30° of the crucible wallsmay allow for easy removal after complete solidification of the moltensilicon 206 (FIGS. 6 and 7) column(s). It is expected that the Grafoilliners will stick to the ingot 206 via a solid layer of NaF 204 if thecrucible is allowed to cool to room temperature.

Consequently, the crucible should not be allowed to cool below about themelting point of NaF so that the resulting silicon 206 (FIGS. 6 and 7)ingot may be readily removed by dumping.

The salt coating 204 remaining on the ingot can be easily removed by aconventional aqueous leaching process. For example, the salt coating 204is readily removed in 1.0N acid solution. For a discussion of aqueousleaching of Si, see copending Sanjurjo patent application Ser. No.337,136 entitled Process and Apparatus for Obtaining Silicon FromFluosilicic Acid filed Jan. 5, 1982 and assigned to the assignee of thepresent invention.

The method of the present invention is advantages in that NaF serves asan impurity sink (i.e., the molten coating salt acts as a purifyingagent). NaF and other salts such as CaF₂, BaF₂, Na₂ SiO₃ and mixturesthereof may be utilized to decrease vaporization and increasepurification of the molten silicon 206 (FIGS. 6 and 7). This effect addsto the versatility of the casting technique. That is, the fact that theadditional purification is obtained allows the casting technique to beused with relatively inexpensive feed grades of Si that would notnormally be considered for a solar grade end product. The salt lining204 or interface may be continuously purged with fresh (purer) NaF toremove any impurities. An additional advantage of the NaF coating 204 isits ability to form a continuous smooth surface between the silicon 206and the crucible wall so as to prevent any irregularities of the wallsurface to affect the crystalization of the silicon 206 ingot.

In accordance with another embodiment of the present invention, coolingof the molten silicon 206 (FIGS. 6 and 7) column(s) is performed at apoint immediately below the liquid-solid silicon interface (asillustrated in FIGS. 6 and 7). As the solidification progresses upwardfrom the bottom, the cooling He gas (inlet 214 and outlet 213) probe(see FIGS. 6 and 7) is raised thus overcoming the disadvantage ofremoving heat through the length of the already solid portion of thesolidified silicon 206 (FIGS. 6 and 7) column(s). The cooling He gasprobe consists of a pair of tubular members open to the inside of thehollow cooling parallelepipeds (235 and 212), one of the pair of tubularmembers constituting a coolant delivery tube (inlet 214) for delivery ofa liquid coolant and the other tubular member of the pair constituting acoolant removal tube (outlet 23) for removal of coolant after deliveryby the delivery tubular member. Seeding and cooling gas probes havingdifferent configurations may also be utilized in carrying out thepresent invention. Other suitable gasses useful in the practice of theinvention are Ne, Ar, Kr, Xe, Rn, or N.

In accordance with another aspect of the present invention, a suitablesalt having an endothermic transition temperature below about 1400° F.is utilized to provide a suitable heat sink for cooling the moltensilicon column (206). The enthalpy of melting for silicon is about 12kcal/mol and for NaF the number is about 7.97 kcal/mol, which means thatabout 67% of the heat of solidification of 1 mol of silicon can beabsorbed by 1 mol of NaF. It is therefore advantages to contact andextract heat from the molten silicon column (206) using a salt material(such as NaF) which is in the solid state and at a temperature wellbelow the melting point of the salt. A typical configuration of the saltmaterial (not shown) useful for removing heat from the molten siliconcolumn (206) may be in the shape of a conical bar (320) (similar to theshape of a rocket nose cone) constructed of compacted NaF powder. Inaccordance with the practice of the instant invention, a substantiallysolid conical bar (320) with a suitably shaped silicon seed embedded atits tip (FIG. 9) is placed within a SiC receiving crucible (322)(similar to a rocket positioned inside a silo with its nose cone aimedupward). The molten silicon (206) column contained within its crucible(200) is then slowly lowered into the upright SiC receiving crucible(322) containing the conical salt bar (320) (similar to a cylindershaped elevator moving downward into a cylinder shaped coal mine shaft).As the crucible containing the molten silicon (206) proceeds downwardinto the SiC receiving crucible (322), the tip of the conical bar comesinto contact with the molten silicon crucible (200) through the bottomorifice. The contact of the seed 208 with the molten silicon (206)result in the melting of the solid NaF conical bar and crystallizationof the molten silicon (206) column.

As the NaF bar melts, it fills the SiC receiving crucible (322) untilthe original NaF bar is completely melted and transformed into a shapeconforming to the shape of the internal SiC receiving crucible (322).The advantages of this method of cooling the molten silicon (206) columnare many. Aside from realizing a lower cost and conservation ofmaterial, the cooling method is more efficient since the total surfacearea of the silicon crucible is utilized for removing heat. It iscontemplated that the rate of solidification and thus the degree ofcrystallinity of the resulting silicon column (206) can be controlled byvarying the shape of the salt bar, the overall geometry of the systemand the rate of any externally applied heat means (328) to the system.Alternatively, heat from the molten silicon column (206) may be removedby a fluidized bed method in which suitable particles of salt insuspension are melted in contact with the bottom of the siliconcrucible. The resulting molten salt may be tapped out or they may beallowed to remain molten and converted back into its solid state byadditional heat extraction using suitable external heat exchangers.

The process sequence shown in FIG. 1 was selected because of theinherent simplicity of the steps and their independent and combinedsuitability for scale-up. Some purification occurs during precipitation(operation 1, FIG. 1) for Mg, Ca, Al, P, and As due to the highsolubility of their fluosilicates and fluosalts. Some concentrationtakes place for Cr, Fe, and Ni, and this effect may be due tocoprecipitation of these elements as fluorides since their fluosilicatesare very soluble. From Table II, it is clear that most of thepurification is accomplished as a result of the thermal decomposition instep 24 (FIG. 1). Most transition metal fluorides are in very stablecondensed phases at the decomposition temperature (850° C.) in step 24(FIG. 1) and, therefore, will stay in the solid. In addition, volatilefluorides formed during the decomposition of fluosalts such as Na₂ TiF₆and Na₂ ZrF₆ will condense upon cooling of the SiF₄ gas stream from step24. The condensed material is then removed from the gas mainstream byin-line fume particle filtration. The presence of any metallic or dopantimpurities was not detected using mass spectrometry (Table I) in eitherthe gas produced in the above reaction or in the commercial SiF₄ gas.The analysis done on the SiF₄ by passing the gas through high puritywater was based on the hypothesis that impurities should be hydrolyzedand/or trapped in the SiO₂ formed.

The results listed in Table II show that the level of metal impuritiesin the resulting SiO₂ is so low that, for practical purposes, the SiF₄can be considered free of metallic impurities. The Na feed, reactormaterials, and possible contamination of the product during handlingremain as possible sources of impurities in the Si.

The impurities in Na can be divided roughly into three types accordingto their tendency to react with SiF₄, as classified by the free energyof reaction. The first type of impurity includes aluminum and elementsfrom the groups IA, IIA and IIIB. The free energy of reaction of SiF₄with these impurities ranges from -100 to -200 kcal/mole SiF₄ at roomtemperature and from -50 to -100 kcal/mole SiF₄ at 1500K. It isexpected, therefore, that even when these impurities are present at theppm level, they will react with the SiF₄ to form correspondingfluorides. Subsequently, the fluorides will be dissolved preferentiallyin the NaF phase.

The second type impurity includes transition metals such as Mo, W, Fe,Co, Ni, and Cu, and the elements P, As, and Sb. These elements exhibitpositive free energies of reaction in excess of 100 kcal/mole SiF₄ andare not expected to react with SiF₄. However, it is an experimental factthat the silicon resulting from the SiF₄ -Na reaction contains amountsof Fe, Ni, and Cr in proportion to the concentration of these elementsin the Na feed. The mechanism by which these metals are transferred tothe silicon has not yet been studied. In any case, the concentration ofFe, Cr, Ni, and also Ti can be decreased by a factor of about 10⁴ to 10⁶for single-pass directional solidification or the Czochralskicrystal-pulling procedures used presently for solar cell manufacture. Atthe resulting levels, these elements would not be detrimental to solarcell performance.

Boron represents a third type of impurity. The free energy of reactionof this element with SiF₄ is positive but small (5-20 kcal/mole SiF₄ fortemperatures up to 1500K); therefore, some partial reaction can beexpected and B will be distributed between the NaF and Si phases. It isnoted that the levels of the dopant elements B,P, and As in the reactionSi are the same as in the semiconductor grade silicon used as referenceor control. Since it is convenient to have dopant levels as low aspossible to permit flexibility in subsequent doping procedures forsemiconductor and solar cell applications, the low B and P content of Siproduced in this process is of advantage. It is noted that the purity ofthe silicon produced by the SiF₄ -Na reaction is, at a minimum,nominally appropriate for solar cell manufacture.

From the foregoing discussion, it will be understood that the objects ofthe invention have been carried in that high purity Si can be preparedand cast using the inexpensive starting materials H₂ SiF₆ and Na.Favorable thermodynamics of the reduction step, easily controlledkinetics, and abundant availability of inexpensive starting materialsmake this method attractive. Of special interest for semiconductorapplications are the low concentrations of B and P impurities in theproduct Si. The Si produced by the SiF₄ -Na reaction, particularly whenpurified further by directional solidification (the casting), should bea low cost material suitable for the manufacture of solar cells andother semiconductor products.

While particular embodiments of the invention have been shown, it will,of course be understood that the invention is not limited thereto sincemany modifications in both process and apparatus employed may be made.It is contemplated that the appended claims will cover any suchmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A system for producing low cost, high purity,solar grade silicon by reaction of gaseous silicon tetrafluoride withsodium in substantially stoichiometric quantities to produce a reactionproduct from which silicon is recovered and wherein said fluoride gasused in the reaction is obtained by thermal decomposition of sodiumfluosilicate which is precipitated from aqueous fluosilicic acidgenerated from phosphate rock conversion to fertilizer, said systemincluding: a chemical reactant feed section and a reactor section; saidreactor section including first and second reactant receivingcompartments having a common wall with passages therethrough of a sizeto allow easy vapor transport; said chemical feed section having firstreactant delivery means to introduce a first reactant into said firstreactant receiving compartment and a second reactant delivery means todeliver a second reactant into said second reactant receivingcompartment; and means to decompose reactant delivered to said secondreactant receiving compartment thereby to generate a vapor phasereactant and cause vapor transport between said second and firstreactant receiving compartments whereby reaction between said vaporphase reactant and reactant delivered to said first reactant receivingcompartment reacts to produce reaction products including silicon, saidreaction products being directed into a reaction product receiving andmelt separation compartment having heating means surrounding saidreaction product receiving and melt separation compartment so as tomaintain said reaction products in the melt and facilitate separation ofsaid reaction products.
 2. A system for producing low cost, high purity,solar grade silicon as defined in claim 1 wherein said reaction productreceiving and melt separation compartment operates to receive saidreaction products produced and operates to separate said silicon fromother reaction products in the melt, said reaction product receiving andmelt separation compartment having walls formed with passagestherethrough, said passages being of a size such that essentially allmolten reaction products other than silicon freely pass through andmolten silicon is preferentially retained.
 3. A system for producing lowcost, high purity, solar grade silicon as defined in claim 2 wherein acasting crucible formed of graphite is provided and said molten siliconis continuously removed from said reaction product receiving and meltseparation compartment, isolated with a molten coating of reactionproducts other than silicon and directly casted into said castingcrucible thereby to cast said silicon directly from the melt, saidmolten coating serves to provide a barrier which prevents said moltensilicon from reacting with said casting crucible, said coating furtherto serve to absorb stress between the walls of said casting crucible andthe solidifying said molten silicon.
 4. A system for producing low cost,high purity, solar grade silicon by reaction of gaseous silicontetrafluoride with sodium in substantially stoichiometric quantities toproduce a reaction product from which silicon is recovered and whereinsaid fluoride gas used in the reaction is obtained by thermaldecomposition of sodium fluosilicate which is precipitated from aqueousfluosilicic acid generated from phosphate rock conversion to fertilizer,said system including: a chemical reactant feed section and a reactorsection; said reactor section including first and second reactantreceiving compartments and third and fourth reaction product receivingcompartments; said first and second reactant receiving compartmentshaving a common wall with passages therethrough of a size to allow easyvapor transport therethrough; said chemical feed section having firstreactant delivery means to introduce a first reactant into said firstreactant receiving compartment and a second reactant delivery means todeliver a second reactant into said second reactant receivingcompartment; means to decompose reactant delivered to said secondreactant receiving compartment thereby to generate a vapor phasereactant and cause vapor transport between said second and firstreactant receiving compartments whereby reaction between said vaporphase reactant and reactant delivered to said first reactant receivingcompartment takes place to produce reaction products; said thirdreaction product receiving compartment disposed to receive reactionproducts from said first and said second reactant receiving compartmentsand having walls formed with passages therethrough, said passages beingof a size such that essentially all molten reaction products other thansilicon freely pass through and molten silicon is preferentiallyretained; said third compartment having heating means surrounding saidreaction product receiving and melt separation compartment so as tomaintain said reaction products in the melt and facilitate separation ofsaid reaction products; said fourth reaction product receivingcompartment substantially surrounding said third reactant receivingcompartment and separated therefrom by said wall of said third reactantreceiving chamber having passages therethrough thereby to collect saidreaction product passing through said passages; said fourth reactantreceiving compartment having an outlet thereby to constitute a reactionproduct collection and dispensing means; and said third reaction productcompartment having an outlet for removing the reaction product remainingtherein by free flow.
 5. A system for producing low cost, high purity,solar grade silicon as defined in claim 4 wherein a casting crucible isprovided and positioned to receive said reaction product from saidoutlet from said third product receiving compartment of said reactorsection whereby the silicon flowing from said reactor section is castdirectly from the melt.
 6. A system for producing low cost, high purity,solar grade silicon as defined in claim 5 wherein said casting crucibleis a high purity graphite crucible lined with liquid salt and having ashape defining a parallelepiped.
 7. A system for producing low cost,high purity, solar grade silicon as defined in claim 6 wherein saidparallelepiped defines a right rectangular prism.
 8. A system forproducing low cost, high purity, solar grade silicon as defined in claim7 wherein means is provided to insert a silicon seed centrally throughsaid bottom wall to contact liquid silicon in said crucible and means tocool the liquid solid interface.
 9. A system for producing low cost,high purity, solar grade silicon as defined in claim 8 wherein means isprovided internally and adjacent said graphite walls to absorb stressbetween solidifying silicon and said graphite walls thereby to absorbstress and prevent fracture of said walls during solidification of saidsilicon.
 10. A system for producing low cost, high purity, solar gradesilicon as defined in claim 5 wherein said bottom wall of said crucibleis provided with a centrally located cooling parallelepiped similar tosaid parallelepiped defined by said lateral faces of said crucible, saidcooling parallelepiped extending upwardly into said crucible from saidbottom wall and sealed therein, said cooling parallelepiped havinglateral faces substantially parallel to corresponding lateral faces ofsaid crucible, and being hollow to provide for cooling from the centerfor silicon in said crucible. during solidification of said silicon. 11.A system for producing low cost, high purity, solar grade silicon asdefined in claim 10 wherein said cooling means includes a cooling probeconsisting of a pair of tubular members open to the inside of saidhollow cooling parallelepiped, one of said pair of tubular membersconstituting a coolant delivery tube for delivery of a liquid coolantand the other tubular member of said pair constituting a coolant removaltube for removal of said coolant after delivery by said one tubularmember.
 12. A system for producing low cost, high purity, solar gradesilicon as defined in claim 10 wherein said side walls of said crucibleare disposed to constitute lateral faces of six parallelepipeds ofhexagonal configuration with each parallelepiped so formed having acommon side wall with each of two adjacent hexagonal parallelepiped andsaid cooling parallelepiped extending up between said sixparallelepipeds to form a similar hexagonal prism whereby said crucibleis structured substantially simultaneously to cast six single crystal orquasi single crystal ingots each in the shape of a hexagonal prism. 13.A system for producing low cost, high purity, solar grade silicon asdefined in claim 11 wherein said cooling means includes a cooling probeconsisting of a pair of tubular members open to the inside of saidhollow cooling parallelepiped, one of said pair of tubular membersconstituting a coolant delivery tube for delivery of a liquid coolantand the other tubular member of said pair constituting a coolant removaltube for removal of said coolant after delivery by said one tubularmember.